A hard disk drive (HDD) spins a magnetic disk of a recording medium at high speed and reads and writes data with a head slider flying and moving over the magnetic disk. An actuator supports a head slider and swings across the magnetic disk to move the head slider along the radial direction of the spinning magnetic disk.
The actuator comprises a suspension secured at the tip of an arm and the suspension supports the head slider. As shown in FIG. 9, a typical suspension 9 comprises a plurality of members and comprises a load beam 91 and a gimbal 92 secured to the magnetic disk side of the load beam 91. The load beam 91 positions the head slider at a specific track in accordance with the operation of the actuator and generates a pressing force to press the head slider toward the magnetic disk. The head slider flies at a specific spacing from the magnetic disk surface under the balance between the pressing force of the load beam and the lift force from the air bearing which is caused by the viscous airflow between the air bearing surface (ABS) and the magnetic disk surface.
The gimbal 92 is generally made of a thin stainless steel and comprises a gimbal tongue (921 in FIG. 10). The head slider is secured to the gimbal tongue. The back side of the gimbal tongue contacts a projected dimple of the load beam 91, and the gimbal tongue and the head slider thereon pivot on the contact point with the dimple. The pivoting motion is known as the pitch and roll motion or the gimbal motion. This movement keeps the spacing and the attitude of the head slider to the magnetic disk surface.
The roll motion is a motion in the direction denoted by R in FIG. 9; specifically a pivot movement on the X-axis which is the longitudinal direction of the suspension 9. The pitch motion is a motion in the direction denoted by P in FIG. 9; specifically a pivot movement on the Y-axis which is perpendicular to the X-axis and included in a surface parallel to the disk surface. The characteristics of the pitch motion and the roll motion are determined by the structure of the entire suspension. Lower pitch stiffness and roll stiffness of the gimbal tongue achieve better track following performance and better flight attitude control.
While a head slider is flying above a track and performing a following motion, the gimbal needs to support the head slider flexibly so that the head slider can maintain the following motion responsive to changes in shapes of the magnetic disk surface. Therefore, it is important to reduce the pitch stiffness and roll stiffness of the gimbal tongue. On the other hand, an HDD is required to have more shock resistance. An HDD with the ramp loading and unloading scheme lifts up the head slider from above the disk surface in unloading and lifts it down to above the disk surface in loading.
To improve the shock resistance of the device and to achieve stable operation where the head slider does not contact the magnetic disk in loading/unloading, it is necessary to take peel stiffness into account in addition to the pitch stiffness and the roll stiffness. The peel stiffness is a vertical stiffness when applying a force to the gimbal tongue normal to the disk surface, which is desirably higher for the shock resistance or loading/unloading operation.
The gimbal 92 functions as a spring mechanism for supporting the slider on the securing point to the load beam as a support point. In a typical structure of a gimbal, if the peel stiffness increases, the pitch stiffness and the roll stiffness increase together, so that a gimbal with higher peel stiffness and smaller pitch and roll stiffnesses cannot be accomplished no matter how parameters for the stiffnesses are adjusted.
In this regard, gimbals having the spring structure with lower pitch and roll stiffnesses and higher peel stiffness have been proposed (for example, refer to Japanese Patent Publication No. 2004-326891 “Patent Document 1”). FIG. 10 is a plan view showing a part of such a gimbal. The gimbal 92 is secured to a load beam at two securing points 922a and 922b sandwiching a gimbal tongue 921 between at the front and at the rear. The securing points 922a and 922b are typically welded by beam welding. Hereinafter, the direction viewing a head slider from the swing shaft of the actuator is defined as frontward and the direction viewing the swing shaft from the head slider is defined as rearward.
In the gimbal 92 of FIG. 10, support arms 923a and 923b protrude from the middle of the side edges of the gimbal tongue 921. The support arms 923a and 923b are connected to the extending frontward main arms 924a and 924b, respectively. The two main arms 924a and 924b join together in the front and the main arms 924a, 924b and the support arms 923a, 923b are continuous. The main arms 924a, 924b and the support arms 923a, 923b form a ring shaped spring structure for supporting the gimbal tongue 921. The main arms 924a and 924b form a path like a big ring swelling vertically to the center line of the gimbal 92.
Moreover, the support arms 923a and 923b are connected to extending rearward sub arms 925a and 925b, respectively. The sub arms 925a and 925b join to a support body 926 in the rear. The sub arms 925a, 925b, the support arms 923a, 923b, and a part of the support body 926 are continuous to form a ring-shaped spring structure for supporting the gimbal tongue 921. The sub arms 925a and 925b extend from the rear toward the front substantially in parallel to the center line of the gimbal 92, then swell, and join to the main arms 924a, 924b and the support arms 923a, 923b. 
In the gimbal, the stiffness of the front ring-shaped spring structure (a main ring MR) including the main arms 924a and 924b is higher than the one of the rear ring-shaped spring structure (a sub ring SR) including the sub arms 925a and 925b, and the main ring MR functions dominantly in supporting the gimbal tongue 921. The pitch stiffness and the roll stiffness of the gimbal tongue 921 (head slider) depend on the shape of the main ring and the sub ring SR contribute to them little.
Specifying parameters of the dominant main ring MR leads to determining the values of the stiffnesses of the gimbal tongue 921, substantially ignoring the sub ring SR's contribution to the stiffnesses. Specifically, reduction of the distance between the gimbal tongue and the securing points achieves higher peel stiffness. Besides, adjustment of values such as the path length, the path shape, and the widths of the main ring achieves smaller pitch and roll stiffnesses, maintaining the higher peel stiffness.
The sub ring (sub arms) affects the stiffnesses a little and it is not the element to determine the values of the above three stiffnesses. The sub arms 925a and 925b are needed in producing a suspension. The sub arms 925a and 925b maintain the entire gimbal in a stable shape in assembling the gimbal and the load beam to improve workability.
The suspension disclosed in the Patent Document 1 achieves lower roll and pitch stiffnesses and higher peel stiffnesses concurrently. This is because, as described above, the sub ring supports the gimbal tongue in an auxiliary manner, and the main ring can dominantly provide the gimbal tongue with stiffness.
The gimbal design in the Patent Document 1, however, considers the pitch, roll, and peel stiffnesses of the gimbal tongue, but does not yaw stiffness. The yaw stiffness is the stiffness to a yaw motion of the gimbal tongue (head slider). The yaw motion is a pivoting motion in the direction denoted by YAW in FIG. 10. Specifically, it is a pivoting motion on the normal to the head slider mount surface (main surface) of the gimbal tongue and is a pivoting motion within the ABS.
Data track pitch significantly decreases with increase in storage capacity of an HDD. Accordingly, a head slider's slight yaw motion causes a head element portion to shift radially from a target position to raise a read/write error. Especially, it has been found that, if the roll stiffness and the pitch stiffness are reduced without considering the yaw stiffness as in the Patent Document 1, the yaw stiffness decreases together to cause oscillation in a yaw mode at low frequency easily.
Increases in the roll stiffness and the pitch stiffness can increase the yaw stiffness, which reduces the following performance of the head slider responsive to change in shape of the magnetic disk surface, however. Accordingly, a suspension is desired that has higher yaw stiffness while keeping higher peel stiffness and lower roll and pitch stiffnesses.